If you have any iTMS purchases the transfer of purchased content from the iPod to authorised computers has been introduced with iTunes 7. A paragraph on it has been added to this article:
Transfer iTunes Store purchases using iPod
The transfer of non iTMS content such as songs imported from CD is designed by default to be one way from iTunes to iPod. However there is a manual method of accessing the iPod's hard drive and copying songs back to iTunes on Windows. The procedure is a bit involved but if you're interested it's posted in this thread:
MacMuse - iPod to iTunes
If you prefer something more automated then there are a number of third party utilities that you can use to retrieve the music files and playlists and in some cases videos from your iPod, this is just a selection. Have a look at the web pages and documentation for these, they are generally quite straightforward. You can read reviews of some of them here:
Wired News - Rescue Your Stranded Tunes
YamiPod Mac and Windows Versions
iGadget Windows Only
iPod Access Mac and Windows Versions
PodUtil Mac and Windows Versions
iPodCopy Mac and Windows Versions
PodPlayer Windows Only
CopyPod Windows Only
If your iPod is set to update automatically take care when connecting back to your computer and an empty iTunes. You will get a message that your iPod is linked to a different library and asking if you want to link to this one and replace all your songs etc, press
"Cancel". Pressing "Erase and Sync" will irretrievably remove all songs from your iPod. Your iPod should appear in the iTunes source list from where you can change the update setting to manual and use your iPod without the risk of accidentally erasing it. Check the "manually manage music and videos" box in Summary then press the Apply button. Don't uncheck Sync Music it will be unchecked by default when you choose the manual setting:
Managing content manually on iPod
Whichever of these retrieval methods you choose, keep your iPod in manual mode until you have reloaded your iTunes and you are happy with your playlists etc then it will be safe to return it auto-sync again. I would also advise that you get yourself an external hard drive and back your stuff up in case you have problems again. External drives are comparatively inexpensive these days and you can get loads of storage for a reasonable outlay.
